Thanks to everybody who participated in this vivid discussion! Consensus was quickly reached on which infraction has been committed: It's a Game Rule Violation.
Vexing Devil should have been put into exile, not into the graveyard; because the effect that caused the infraction is controlled by Amun, but the illegal action is taken by Nefertiti, they both receive a Warning for Game Play Error – Game Rule Violation. Donato was very fast in pointing this out.
Backing up is something the Head Judge can decide to do, but is not an ideal solution, as Robert Hinrichsen explained. A lot of decisions have been made since the error occured (activate
Gideon Jura, attack with everything, cast
Tragic Slip) and this probably will affect the course of the game.
When not backing up (which isn't perfect either), we don't apply any kind of partial fixes, but the IPG provides 3 exceptions to this rule: “If an object is in the wrong zone, and it is within a turn cycle of the error, put the permanent in the correct zone.”
Vexing Devil should be put into exile from the graveyard.
Now, if
Tragic Slip resolves, it checks wether a creature has died this turn. This was the main discussion point! Although
Vexing Devil was put into a graveyard, it didn't technically die, it just was put there accidentally. So Morbid does not apply and
Tragic Slip only gives Gideon -1/-1 until end of turn, just as Donato stated.
So, what is our final verdict? That there are situations like these that don't have a clear best way to handle them! You as Head Judge should decide on the spot whether you want to back up or apply a partial fix, and whatever you decide, it is correct and justified by the IPG. Use your judgment, that's what you are a Judge for. Consensus was also reached among the Knowledge Pool team; most of us would not back up, because of the number of decisions that have affected the game.
See you soon in the next edition of the Knowledge Pool!